Precise alignment of adjacent sections of a mold is critical to the production of a precision-molded part such as a lens. The alignment of adjacent sections of a mold is most challenging when the adjacent sections are positioned across the parting line of the mold. Once the mold is aligned a molded part may be produced with the accuracy and tolerances required by the designer. For example, it is envisioned that some applications will require an alignment precision of less than ½ a micron.
During the molding operation, the accuracy of the alignment between adjacent sections of the mold determines the accuracy of the optical surfaces of the molded lens. The performance of the molded lens is often limited by the accuracy of the alignment of the optical surfaces of the lens relative to each other. Misalignment of the axis of one surface of a lens relative to the axis of the other surface of the lens is the major contributor to lens irregularity problems such as coma and RMS. Ultimately, the lens problems impact the imaging quality of the lens. In many applications the axis of each surface of the lens cannot be misaligned by more than 10 microns. In some cases the misalignment cannot exceed 3 microns.
Misalignment of the optical surfaces in the mold cavity results in the misalignment of the axes of the surfaces of the molded lens. Specifically, metal inserts in the mold that include the optical surfaces are rod-like structures known as nubbins. When the mold halves are brought together, the nubbins form a cavity with optical surfaces inside the mold. Molten plastic is then introduced in the mold cavity under pressure such that it conforms to the shape of the cavity and replicates the optical surfaces of the mold to form a lens. The plastic is then cooled sufficiently to enable the molded lens to be removed or ejected from the mold cavity. As a result, when the nubbins are not properly aligned, the axis of each side of the lens is misaligned.
Conventional methods for aligning the surfaces of a lens in a mold include using guide pins and/or taper locks positioned within a mold. The guide pins and/or taper locks are used to enable one half of the mold to accurately engage a second half of the mold.
Since the guide pins must slide together when the mold halves are brought together, they must have sliding clearance. The clearance requirement for the guide pins is about 5 microns for each sliding clearance. Multiplying this by the number of guide pins and combining this for both mold halves, results in a 10 to 20 micron misalignment problem resulting from the guide pins.
A number of studies have shown that the nubbin clearance is a major contributor to misalignment in a molded plastic lens. Nubbin alignment in plastic lens molding is difficult to control due to the need for clearance around a first nubbin (i.e., found in the first half of the mold) that enables the first nubbin to move during ejection of the molded lens after molding. Typically, the clearance required of the first nubbin is on the order of 3 to 5 microns and a similar clearance is required of the oppositely disposed nubbin (i.e., on the opposite side of the mold). The total typical misalignment for a molded plastic lens is then 16 to 30 microns based on the guide pins and the nubbins.
In US 2001/0053395 Hosoe describes a method for aligning molding die members wherein a flowing gas or liquid is supplied to the clearance around a nubbin inside a bore in a mold. As a result, the nubbin is forced into the center of the bore.
However, the design in Hosoe is complex. The design requires a uniform flow of gas on both sides of the nubbin or the nubbin will be preferentially pushed to one side or the other. In addition, since the control of the flowing gas is complex, the piping disclosed in Hosoe is elaborate and difficult to include in a mold.
Thus, there is a need for a method and apparatus for aligning a nubbin. There is a need for a method and apparatus for aligning each section of a mold relative to the other.